Arriving and departing aircraft must travel on the ground between landing and subsequent takeoff along runways and taxiways associated with an airport. Over time, the tarmac surface of a runway or taxiway can change as a result of rubber buildup from the application of aircraft brakes, paint used for marking, chemicals, and erosion of the tarmac material. These changes in the runway surface produce changes in the friction between the runway surface and the aircraft's tires. The presence of moisture, whether from rain, slush, snow, or ice, however, is a major factor in the degradation of runway surfaces. At a minimum, braking action is diminished, and a longer landing distance is required.
Aircraft tires may also become stuck to the runway surface under adverse ground conditions. The coefficient of runway friction or slipperiness, μ, is theoretically 1 when the runway friction characteristics are 100%. For a runway with 0% friction characteristics, μ=0. All other friction characteristics fall between 0% (μ=0) and 100% (μ=1). Most new runways have a coefficient of friction of about μ=0.6 (60% friction characteristics). A coefficient of friction μ>0.4 (greater than 40% friction characteristics) is generally considered to be good. The factors mentioned above all contribute to runway deterioration and decrease the coefficient of friction, affecting aircraft braking and landing distance. The presence of moisture, particularly in the form of snow or ice, can have a significant effect on the friction characteristics between an aircraft's tires and the runway surface. This can lead to a situation in which the frictional forces between the tire and the runway cannot be overcome by direct aircraft pressure, resulting in a stuck aircraft with one or more tires adhered to the runway surface.
When an aircraft is completely stopped and at rest on the ground, there are numerous factors in addition to friction characteristics that can make movement of the aircraft from this resting condition difficult, particularly in cold weather. For example, tires that are cold tend to become misshapen, making them harder to turn. When a tire has become flattened where it contacts the tarmac, the force required to move the aircraft includes the force needed to lift the aircraft over the misshapen tire. Aircraft tires can also become stuck to the tarmac when water freezes between the tire and the tarmac or through light adhesion between the tire and tarmac in drier conditions. In winter conditions, snow and slush buildup can exacerbate the situation. In addition, when the aircraft wheel bearings are cold, they are more resistant to movement than when the bearings are warm, adding an additional frictional force to be overcome. Under these conditions, the force required initially to move an aircraft from a resting condition to a moving condition can be much greater than the force required to keep the aircraft moving, once frictional forces and inertia have been overcome and movement has started.
Aircraft are most often n immobile after arrival, when they are parked at a gate or other docking structure. The time an aircraft is required to spend at a gate will depend, in part, on the turnaround schedule. Some aircraft have longer turnaround times than others. In inclement weather, especially when the temperatures are around freezing, the likelihood of ice forming between one of more of the aircraft tires and the tarmac can be quite high, causing the tires and, hence, the wheels to become stuck to the tarmac.
Methods and apparatus for reducing the adhesion between ice on a travel surface and an object traveling on the surface are known. U.S. Pat. No. 7,034,257 to Petrenko et al, for example, proposes a method to modify friction between an object and ice or snow that is suggested to be applicable to aircraft landing gear. This method employs a heating element to apply a pulse of thermal energy to melt ice at the interface of an object and the ice or snow. While this method may be effective in other applications, it involves having available additional equipment and additional ground personnel to use the equipment to free a parked aircraft stuck to ice and get the aircraft moving. U.S. Pat. No. 7,743,653 to Stommel describes a method of adapting tires of aircraft and other vehicles to travel surface conditions by changing the shape of the tire to increase or decrease contact between the tire and the travel surface, thereby increasing or decreasing friction between the tire and the travel surface, by raising or lowering tire pressure in response to a sensed travel situation. Stommel, however, does not even remotely suggest that this system would be effective or could be used in snow, ice, or other conditions to release an aircraft tire that has become stuck, directly or indirectly, to the travel surface.
An aircraft with one or more tires immobilized by ice presents challenges during push back when a tow vehicle or tug is used. The weight of the tug helps to apply sufficient force to overcome the frictional, inertial, and other forces keeping the aircraft stuck in the ice. Aircraft equipped with self-propelled drive wheels powered by electric drivers, such as the system disclosed in U.S. Patent Application No. 2009/0114765 to Cox et al, work very effectively to move aircraft on the ground without external-assistance under almost all environmental conditions. These systems, however, are functionally required to be small and lightweight and cannot apply the same force as a tug to free aircraft wheels stuck in ice. Increasing the size of the driver in a powered self-propelled aircraft wheel to provide more force directly to the wheel to overcome ice adhesion is not a viable solution because these systems must remain as small and lightweight as possible, in part to fit within the space allotted for an aircraft's landing gear.
It would be highly desirable to be able to fully utilize the benefits of a powered self-propelled aircraft drive wheel, particularly at push back, under all types of runway and environmental conditions, especially those which cause adhesion of the aircraft's tires to the tarmac. The prior art has not provided a method for operating a powered self-propelled aircraft drive wheel under adverse runway conditions which have caused one or more of the aircraft's tires to adhere directly or indirectly to the tarmac that employs the powered self-propelled drive wheel to release the aircraft tires and enable the aircraft to move.